It would help to be able to see it.

But it's likely that your dates are being perceived as text, and therefore the formatting is not applying correctly. Perhaps you have some spaces or other invisible characters in your date cells, or maybe, especially if they've been pasted or extracted in from text, they just need refreshing.

You can do this a couple of ways. You can highlight the column, and use the Text to Columns button on your Data ribbon. Use Delimited by Tab, as there won't be any tabs inside the cell, so you won't be parsing on text by accident. That should do it.

You can also use a helper column next to it and use the formula "=VALUE(A2)" to convert the text date to a number, and reformat that.

But the down-dirty way of doing it for me (I used to work with huge sheets of exported numbers stored as text) was to use Find/Replace. For me, I didn't have dates, so I had to replace every integer from 0 to 9 with itself, which I did with a simple macro. Try highlighting the column with the dates in it and hit Find/Replace. If they're dates from this year or last year, replacing every 2 with a 2 (in other words, making no material changes but causing the cell to be refreshed) should touch every cell.

Best,

Ann